When a fuel cell stack assembly is operating normally and there is a sudden increase in the load, the capability of the fuel cell stack assembly to provide the required current is primarily a function of the amount of fuel resident in the fuel flow fields. If the fuel flow fields are designed so as to have a larger volume of fuel adjacent to the fuel cell membrane, the fuel cell stack assembly will have a greater increasing-load transient response capability. However, providing increased fuel volume in the fuel flow fields requires that the fuel channels be larger at the expense of thinner inter-channel ribs, which will decrease the strength of the fuel cell stack, and can lead to cracking of the fuel cells during assembly and other handling. On the other hand, providing a larger volume for each fuel cell would usually be prohibitive in automotive applications, such as where a fuel cell power plant provides the electricity for an electric vehicle.
In order to achieve maximum fuel utilization, on the order of 90% or more, fuel recycle may be employed. In such a case, there is essentially no excess hydrogen resident in the fuel flow fields. Thus, the use of fuel recycle to boost system efficiency comes at the expense of a reduction in increased-load transient capability.
A fuel cell power plant operating normally may undergo a sudden decrease in load. The fuel processing system, including valves and solenoids, lags in its response to this reduction in demand, so that a large amount of excess hydrogen is not consumed in the fuel cell stack. The excess hydrogen may pass into a downstream burner of some sort; the amount of hydrogen fed to the burner under a reduced-load transient may be several times the amount of hydrogen that the burner would normally have to handle. In order to be able to consume the excess hydrogen, without causing equipment damage, the volume and burning capacity must be increased, which increases the overall volume and cost of the fuel cell power plant. If excess hydrogen is simply exhausted, it must be done so in a safe manner, such as by diluting the hydrogen to less than one percent; this may require increased ventilation capabilities in the entire fuel processing system. Tighter controls on the fuel control response rate may mitigate the problem of excess hydrogen, but it would never eliminate the problem entirely, and may add significant cost to a fuel cell system.
The problem of excess fuel resulting from a sudden decrease in load is referred to in U.S. Pat. No. 6,572,993. However, the system therein operates a storage device, such as a battery pack, a plurality of capacitors or a plurality of supercapacitors, at a nearly uniform level which is a predetermined percentage of the maximum charge level, such as 80%. In response to a sudden reduced load, the energy storage device absorbs excess energy generated by the fuel cell until the amount of fuel entering the anode is lowered to a level not to require the energy storage device to be necessary. At that time, the energy storage device dissipates the excess energy back to its original predetermined level. When the load returns to its previous, normal level, the energy storage device provides excess energy to maintain a desired power output level. As more fuel is provided to the anode, the storage level of the electric power and the energy storage device can be returned to its normal level.
The foregoing patent, therefore, has a system in which steady state conditions are paramount, and transient dips in the load are accommodated by the manner in which the energy storage device is controlled to absorb excess energy, and to thereafter dissipate it, and at the end of the transient provides excess energy and is thereafter returned to its original predetermined level.
If capacitors or supercapacitors are used as the electric storage device, since charge is equal to the voltage times the capacitance, and the charge is always set to a specific fraction, such as 80% of maximum charge, the system of said patent is always set to a fixed, certain, specific predetermined voltage, regardless of anything else. If batteries are used, the voltage will be constant, at any given level of charge.
The aforementioned system will not satisfy the requirements of most modern applications of a fuel cell power plant upon which the demand varies continuously and significantly, such as a fuel cell power plant providing the power to an electric vehicle.